I guess, to me, it's a difference in familiarity. It's like seeing someone in person who, for example, did or said something nice. If it were a person I knew and felt close to I might hug them. But if it was someone I'd just met, or didn't know well, I'd simply say thanks. I'm not a hugger in real life. (Actually, I'm mostly a hermit and hardly know anyone besides my spouse.) So for me, both in real life as well as on-line, hugs is something I give out sparingly.